The Sun

Sun Fact Sheet The Sun is a normal G2 star, one of more than 100 billion stars in our galaxy. Diameter: 1,390,000 km (Earth 12,742 km or nearly 100 times smaller) Mass: x kg (333,000 times Earth’s mass) Temperature: 5800 K (surface) 15,600,000 K (core) The Sun contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest). Chemical composition: Hydrogen 92.1% Helium 7.8% Rest of the other 90 naturally occurring elements: 0.1%

The Sun and its Planets to Scale

Energy is created in the core when hydrogen is fused to helium. This energy flows out from the core by radiation through the radiative layer, by convection through the convective layer, and by radiation from the surface of the photosphere, which is the portion of the Sun we see.

The seasons occur because the tilt of the Earth's axis keeps a constant orientation as the Earth revolves around the Sun. A. Summer in northern hemisphere. B. Winter in southern hemisphere

Sun does not rotate as a rigid sphere. The equator of the Sun rotates faster than the poles of the Sun. This is called the differential rotation. Sunspots and many other solar activities are due to this differential rotation.

Internal Rotation False color image showing a theoretical model of relatively hotter (red) and colder (blue) regions in the solar interior. The red layer may be a shear region between the radiative and convective zones, powering a dynamo that gives rise to the Sun’s magnetic field.

Sun’s Magnetic Field The Sun's corona is threaded with a complex network of magnetic fields. Solar storms and flares result from changes in the structure and connections of these fields. When some of the Sun's magnetic field lines are filled with hot gas, we see a magnetic loop.

The most rapid changes to the Sun's magnetic field occur locally, in restricted regions of the magnetic field. However, the entire structure of the Sun's global magnetic field changes on an 11 year cycle. Every 11 years, the Sun moves through a period of fewer, smaller sunspots, prominences, and flares - called a "solar minimum" - and a period of more, larger sunspots, prominences and flares - called a "solar maximum.“ After 11 years, when the next cycle starts, the magnetic field poles are reversed. The last solar minimum was in 2006 X-ray images of the Sun taken by the Yohkoh spacecraft, showing changes in the corona in 1991 (left) at a solar maximum to 1995, a solar minimum (right).

Sunspots Sunspots appear as dark spots on the surface of the Sun. Temperatures in the dark centers of sunspots drop to about 3700 K (compared to 5700 K for the surrounding photosphere). They typically last for several days, although very large ones may live for several weeks.

Spectrum analysis shows that sunspots have strong magnetic field, about 1000 times stronger than the Sun's average. Sunspots usually appear in pairs. The two sunspots of a pair have different polarities, one would be a magnetic north and the other is a magnetic south, and can be joined by magnetic field lines. The strong magnetic field locks the gas of the photosphere in places and inhibits the hotter gas below to rise at the sunspots. As a result, the sunspots are cooler. Sunspots appear to coincide with changes in the climate of the Earth. Studies show that during the last ice age, there were very few sunspots

Temperature of the Sun’s Atmosphere

Prominences are dense clouds of material suspended above the surface of the Sun by loops of magnetic field. Prominences can remain in a quiet or quiescent state for days or weeks. However, as the magnetic loops that support them slowly change, prominences can erupt and rise off of the Sun over the course of a few minutes or hours Solar Prominences

Solar Flares Images from SOHO* *NASA/ESA Solar and Heliospheric Observatory spacecraft Solar flares are tremendous explosions on the surface of the Sun. In a matter of just a few minutes they heat material to many millions of degrees and release as much energy as a billion megatons of TNT. They occur near sunspots, usually along the dividing line (neutral line) between areas of oppositely directed magnetic fields.

Coronal Mass Ejections (CMEs) CMEs disrupt the flow of the solar wind and produce disturbances that strike the Earth with sometimes catastrophic results. Coronal mass ejections (CMEs) are huge bubbles of gas threaded with magnetic field lines that are ejected from the Sun over the course of several hours.

Corona and Solar Wind Solar wind is the continuous flow of charged particles (ions, electrons, and neutrons) that comes from the Sun in every direction. Solar wind consists of slow and fast components. Slow solar wind is a consequence of the corona’s high temperature. The speed of the solar wind varies from less than 300 km/s (about half a million miles per hour) to over 800 km/s. The Sun’s Corona is forever expanding into interplanetary space filling the solar system with a constant flow of solar wind.

Solar wind shapes the Earth's magnetosphere and magnetic storms are illustrated here as approaching Earth. These storms, which occur frequently, can disrupt communications and navigational equipment, damage satellites, and even cause blackouts. The white lines represent the solar wind; the purple line is the bow shock line; and the blue lines surrounding the Earth represent its protective magnetosphere.